Ferromagnetic mn-ga alloy and method of production



United States Patent ()fi ice 3,147,112 Patented Sept. 1, 1964 3,147,112FERROMAGNETIC Mir-Ga ALLOY AND METHOD OF PRODUCTION Torn Allen Either,.lr., Wilmington, DeL, assignor to E. I.

du Pont de Nemours and Company, Wilmington, DeL,

a corporation of Delaware No Drawing. Filed Jan. 19, 1961, Ser. No.83,638

9 Claims. (Cl. 75134) This invention is concerned with new ferromagneticmaterials and methods for their preparation. More particularly, thisinvention is concerned with ferromagnetic manganese-gallium compositionsand with methods for their preparation.

Various ferromagnetic alloys are known which derive magnetic propertiesfrom the presence of manganese. Among these are ternary Heusler alloysas well as such binary compositions as manganese bismuthide. Thesecompositions, especially manganese bismuthide which possessesoutstanding characteristics as a permanent magnet material, have foundutility in magnetic applications. However, in spite of the fact thatmanganese is responsible for the magnetic properties of these products,the manganese content thereof is relatively low.

The present invention provides highly useful, manganese-richferromagnetic compositions consisting essentially of 55-70 atom percentmanganese and 45-30 atom percent gallium. These compositions include 1)compositions having a tetragonal crystal structure and (2) compositionshaving a cubic structure.

The tetragonal manganese-gallium compositions of this invention areprepared by fusing a mixture of manganese and gallium in the desiredproportions within the above stated range, solidifying the molten massby cooling, annealing the solid at a tempreature of 500-600" C., andquenching. The solid, before annealing, exhibits a cubic crystalstructure, consists essentially of 55-70 atom percent manganese and45-30 atom percent gallium, and is ferromagnetic at low temperatures.This cubic manganese-gallium composition is a feature of the presentinvention.

The properties of tetragonal manganese-gallium compositions depend uponthe proportions of manganese and gallium employed. For example, Curietemperature increases with increasing manganese content from about 385C. for the 55 :45 (manganese: gallium) composition to about 475 C. forthe 70:30 composition. The magnetic moment per gram or sigma valuemeasured in a field of 4000 oersteds also varies with themanganesezgallium ratio and is a maximum in the neighborhood of the60:40 composition. Somewhat higher proportions of manganese in the rangeof 65-67 atom percent produce compositions having high remanence. At theextreme limits of the broad composition range, i.e., formanganesezgallium ratios of 70:30 and 55:45, the presence of minoramounts of other phases besides tetragonal phase becomes apparent. Inorder to avoid these other phases, it is preferred that the proportionof manganese: gallium be in the range of 68 :32 to 60:40.

The cubic manganese-gallium compositions of this invention are obtaineddirectly by solidfying the melt and can be stabilized by cooling thesolid rapidly to a temperature below 400 C. The cubic composition canalso be obtained by annealing the tetragonal composition at atemperature of 800 C. or above and then cooling rapidly. The propertiesof the cubic composition like those of the tetragonal composition,described above, depend upon the relative proportions of manganese andgallium present. For example, Curie temperature for the cubiccomposition increases with increasing manganese content from about -l20for the 55:45 composition (manganese:gal-

lium) 'to about -60 C. for the 70:30 composition. Compositionscontaining 60-68 atom percent manganese and 40-32 atom percent galliumare particularly desirable. The sigma value (0 for the cubiccompositions is very high.

In preparing the compositions of this invention, manganese and galliumin the desired proportions, are thoroughly mixed and placed in acontainer, e.g., an alumina crucible or in a quartz tube. If desired,the container may be hermetically sealed. The container and contents areheated to a temperature and for a time sufficient to melt theingredients completely. This temperature will usually be in the range of1000-1400 C. Prolonged heating is unnecessary provided complete meltingis achieved. Heating is carried out in an inert surrounding environment,such as: vacuum, in which case the pressure will not exceed theautogenous pressure of the reactants; or an inert atmosphere such asargon or helium. When an inert atmosphere is employed the pressure willbe approximately atmospheric pressure, i.e., in the range of 0.5-1.5atmospheres.

After complete melting has occurred, the composition is caused tosolidify by cooling to a temperature below the melting point, usually to700-900 C. When it is desired to prepare the cubic manganese-galliumcompositions, the solid is then cooled rapidly to a temperature below400 C. When, on the other hand, the tetragonal manganese-galliumcompositions are prepared, the solid is cooled to a temperature in therange of SOD-600 C. and held within this temperature range untiltransformation to the tetragonal phase has occurred. Usually this annealwill require from 1-200 hours. After annealing, the tetragonalcomposition is cooled rapidly, i.e., quenched, to a temperature below400 C. As indicated above, the tetragonal phase can be converted to thecubic phase by annealing at a temperature of 800-900" C., followed byquenchmg.

The time required for quenching is related to the amount of material.While very short times, i.e., a few seconds, can be achieved with smallobjects, longer times are required for larger objects. Quenching timeusually will not exceed 1-2 minutes, or, in other words, the rate oftemperature decrease during quenching will preferably be no less thanC./minute. It is preferred that quenching be carried out in less thanone minute.

The examples below are illustrative of this invention.

In these examples commercially available materials are employed andquantities are given in parts by Weight except as noted. The magneticproperties described are the magnetic moment per gram measured in a 4000oersted field (0 the remanence (tr the remanence ratio (a za the Curietemperature and the intrinsic coercive force, H These properties aredescribed in Ferromagnetism, by Bozorth, D. Van Nostrand Co., Inc., NewYork, 1951, pp. 5-8. The sigma values given herein are determined onapparatus similar to that described by T. R. Bardell on pp. 226-228 ofMagnetic Materials in the Electric Industry, Philosophical Library, NewYork, 1955. Values for coercive force herein are determined on a DC.ballistic-type apparatus which is a modified form of the apparatusdescribed by Davis and Hartenheim in the Review of ScientificInstruments, 7, 147 (1936). The Curie temperature, T is determined frommeasurements of the sigma value (0 at various temperatures.

EXAMPLE I A blend of 2.20 g. of manganese powder and 1.39 g. of galliumcut in small pieces (Mn:Ga atom ratio, 66.71333) contained in a quartztube was heated to 200 C. during 40 minutes. In the course of thisheating, the tube was alternately evacuated and flushed with argon toeliminate air. Heating was then continued under argon to a temperatureof 1150 C. which was attained after 4 hours. The sample was maintainedat 1150 C. for 3.5 hours, whereupon the furnace was turned oil and thesample, still in the furnace, allowed to cool. The product after removalfrom the quartz tube was a very hard, metallic slug which upon fractureshowed a silvery interior.

This product was pulverized by grinding and the powder separated intotwo fractions by exposure to a magnetic field (magnetic separation) atroom temperature. One fraction so obtained was strongly magnetic at roomtemperature and possessed a Curie temperature of 470 C. The otherfraction was non-magnetic at room temperature but became stronglymagnetic on cooling. The Curie temperature of this second fraction was66 C.

In a second preparation, a composition containing manganese and galliumin the above proportions was prepared by heating the ingredients in asealed and evacuated quartz tube to a temperature of 1107 C. over a9-hour period, maintaining this temperature for 16.5 hours, coolingslowly to 837 C. during a 7-hour period and finally cooling to roomtemperature. This product also contained magnetic materials of high andlow Curie temperature. X-ray diffraction analysis showed the material oflow Curie temperature (-66 C.) to have a cubic crystal structure and thematerial of high Curie temperature to have a tetragonal crystalstructure.

In a further preparation of the above manganese-gallium composition, theingredients were heated in vacuo to 1092 C. over a 9-hour period, heldat this temperature for 14 hours, slowly cooled to 700 C. over a7.5-hour period, and finally cooled to room temperature. The materialwas pulverized to a particle size below 80 mesh and magneticallyseparated at room temperature to remove approximately of the sample. Theremaining 90% was a strongly magnetic material having a Curietemperature of 475 C. The X-ray diffraction pattern of this materialshowed it to have a tetragonal crystal structure. For determination ofmagnetic properties, a portion of the material was heated to 500 C. andslowly cooled to room temperature in a 5000 oersted field. The magneticproperties (measured in a field of 4000 oersted) were as follows: 0 28gauss cm. /g.; J 25 gauss cm. /g.; remanence ratio, 0.89; intrinsiccoercive force, 3060 oersteds. Analysis of the material showed Mn,59.91; Ga, 37.62. (MnzGa atom ratio, 668733.13).

EXAMPLE II A mixture of manganese and gallium (atom ratio, 60:40) wasplaced in a quartz tube, closed at one end, which was in turn placed ina larger quartz tube. After evacuation, the outer tube was sealed, andthe assembly heated to 1051 C. over a 6-hour period. Heating wascontinued to a temperature of 1090 C. during 15 hours. The assembly wasthen slowly cooled to 850 C. during 8.75 hours, and finally allowed tocool to room temperature. The product was a silvery metallic slug whichwas pulverized to a fine powder by grinding. This powder was shown byX-ray diffraction to consist of a cubic phase, present in major amount,and a tetragonal phase. The respective Curie temperatures wereapproximately 110 C. and 390 C.

A portion of the unseparated powder was pelleted, sealed in a quartztube under vacuum, and annealed at a temperature of 843 C. for 71 hours.At the conclusion of this period, the pellet was quenched in ice water.X-ray diffraction analysis showed that the annealed pellet now consistedpredominantly of the cubic phase. The Curie point measured on thispellet was 120 C.

A second portion of the above powder, pelleted and sealed in a quartztube under vacuum, was annealed at 576 C. for 135 hours and thenquenched in ice water. This pellet exhibited an X-ray diffractionpattern characteristic of the tetragonal crystal structure and had aCurie temperature of 390 C. After cooling slowly from a temperature of500 C. in a field of 5000 oersteds, the pellet had the followingmagnetic properties: 0 37.6 gauss cm. /g.; e 19.2 gauss cm. /g,;intrinsic coercive force, 1150 oersteds.

In another preparation of the manganese-gallium composition (MnzGa atomratio, 60:40) carried out as described above, the product, aftergrinding and pelleting, was annealed at 560 C. for 162 hours in a quartztube under vacuum, and quenched in ice water. The annealed pellet wascomposed entirely of crystals having a tetragonal structure as shown byits X-ray diffraction pattern and exhibited a Curie point of 390 C. TheX-ray difiraction pattern indicating a tetragonal structure with cellconstants: a =2.75 A.; c =.64 A., is shown in Table I below.

Table I X-RAY DIFFRACTION PATTERN OF TETRAGONAL PHASE (60:40 Mn:Ga)

Intensity 1 Interplanar Intensity 1 Interplanar spacing 2 spacing 2 1 Sindicates the strongest line in the pattern, M|, N13, M3 and M4 indicatelines of moderate intensity (decreasing in the order M1 to M4), Findicates faint lines, and V indicates very faint lines.

2 In Angstrom units.

EXAMPLE III A composition containing manganese and gallium in theproportions 62.5:37.5 (atom ratio) was prepared by the general proceduredescribed in Example II. The mixture was heated to a temperature of 1078C. during 8.25 hours, then slowly heated to 1105 C. during 15.25 hours,slowly cooled to 700 C. over 8.67 hours, and finally cooled to roomtemperature. The silvery metallic slug obtained was pulverized. Theproduct was strongly magnetic at low temperature and exhibited a Curietemperature of 65 C. The magnetic moment per gram (0 measured at -l C.was 49 gauss cm. /g.

The X-ray diffraction pattern of this product showed that thepredominant constituent was material having a cubic crystal structurewith a =9.03 A. A little material of tetragonal structure was alsopresent. The X-ray diffraction pattern of the cubic material is shown inTable II.

Table II X-RAY DIFFRACTION PATTERN OF CUBIC PHASE (62.51375 MnzGa)Intensity 1 Interplanar Intensity 1 Interplanar spacing 2 spacing 2 Sindicates the strongest line in the pattern, M1, M M3 and Mt indicatelines of moderate intensity (decreasing in the order M l to Mr), Findicates faint hues, and V indicates very faint lines.

2 In Angstrom units.

EXAMPLES IV-VI These examples illustrate the preparation of variousmanganese-gallium compositions. The products were prepared by heatingthe appropriate mixture of manganese and gallium, contained in analumina crucible, in an atmosphere of argon under about 0.6 atmospherepressure.

The mixtures employed and the heating conditions used are shown in TableIII. The manganese-gallium composition was initially heated to thetemperature shown under the heading Heating, during the period of timeindicated. The composition was next cooled to the temperature shownunder the heading Cooling, in the time indicated and finally cooled,while still in the furnace, to room temperature. The product was thenannealed at a temperature of 560-570 C. for the time indicated andquenched in ice water. The properties of the products are also shown inTable III.

solid to a temperature below 400 C.; cooling finally to roomtemperature; and isolating the resulting solid ferromagneticcomposition.

6. Process for the formation of a ferromagnetic composition consistingessentially of 55-70 atom percent manganese and 45-30 atom percentgallium, having substantially the tetragonal crystal structure whichcomprises: mixing the desired portions of manganese and gallium, heatingthe mixture in an inert environment to a temperature in the range ofapproximately 1000-l400 C. to produce complete fusion; solidifying themixture by Table III PREPARATION AND PROPERTIES OF MANGANESE-GALLIUMCOMPOSITIONS Heat-treatment; Properties of products 1 MnzGa Exampleratio Heating Cooling Anneal- Coercive Magnetic Curie N 0. (atom ingtime force, Hci moment, temp.,

percent) (hrs.) (oersteds) moo (gauss T 0.)

Temp. Time Temp. Time emfl/g.)

( 0.) (min) C.) (min) 2 Remanence ratio was 0.73.

The manganese-gallium compositions of this invention cooling to atemperature in the range of approximately are prepared by simpleprocesses and provide strongly magnetic materials whose propertiesrender them suitable for use in fabrication of permanent magnets. Themassive compositions are readily machined and, after grinding to powder,can be readily fabricated into intricate shapes by powder metallurgytechniques. The compositions of cubic crystal structure are particularlysuited for applications requiring a material strongly magnetic at lowtemperatures but non-magnetic at ordinary temperatures.

As many apparently widely different embodiments of this invention may bemade Without departing from the spirit and scope thereof, it is to beunderstood that this invention is not limited to the specificembodiments thereof except as defined in the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows.

I claim:

1. Ferromagnetic compositions consisting essentially of 55-70 atompercent manganese and 45-30 atom percent gallium.

2. Ferromagnetic compositions consisting essentially of 55-70 atompercent manganese and 45-30 atom percent gallium, having a tetragonalcrystal structure, and having a Curie temperature in the range 385 C. to475 C. measured in a field of 4000 oersteds.

3. Ferromagnetic compositions consisting essentially of 55-70 atompercent manganese and 45-30 atom percent gallium having a cubic crystalstructure.

4. Ferromagnetic compositions consisting entirely of a tetragonalcrystal structure, consisting essentially of 60-68 atom percentmanganese and 40-32 atom percent gallium, and having a Curie temperaturein the range 385 C. to 475 C. measured in a field of 4000 oersteds.

5. Process for the formation of a ferromagnetic composition consistingessentially of 55-70 atom percent manganese and 45-30 atom percentgallium which comprises: mixing the desired proportions of manganese andgallium; heating the mixture in an inert environment to a temperature inthe range of approximately 100'0-1400 C. to produce complete fusion;cooling first to a temperature of 700-900 C. to solidify the melt;cooling the 700-900' C.; cooling the solid to a temperature in the rangeof approximately 500-600 C. and annealing at this temperature for 1 to200 hours until transformation to the tetragonal phase has occurred;quenching at a cooling rate of at least C. per minute to a temperaturebelow 400 C. and isolating the resulting ferromagnetic com position.

7. Process for the formation of a ferromagnetic composition consistingessentially of 55-70 atom percent manganese and 45-30 atom percentgallium having substantially the cubic crystal structure whichcomprises: mixing the desired proportion of manganese and gallium;heating the mixture in an inert environment to a temperature in therange of approximately 1000-1400 C. to produce complete fusion;solidifying the mixture by cooling to a temperature in the range ofapproximately 700-900 C.; cooling the solid at a rate of at least 100 C.per minute to a temperature below approximately 400 C., and isolatingthe resulting ferromagnetic composition.

8. Ferromagnetic compositions consisting essentially of 60-68 atompercent manganese and 40-32 atom percent gallium having a cubic crystalstructure.

9. Ferromagnetic compositions consisting essentially of approximately66.7 atom percent manganese and approximately 33.3 atom percent gallium.

References Cited in the file of this patent UNITED STATES PATENTS846,979 Churchward Mar. 12, 1907 2,202,012 Long May 28, 1940 2,230,236Dean Feb. 4, 1941 2,264,038 Howe Nov. 25, 1941 2,534,178 Marquaire Dec.12, 1950 OTHER REFERENCES Hansen: Constitution of Binary Alloys, SecondEdition, 1958, McGraw-Hill Book Co., Inc., page 748 relied on.

Zeitschrift fiir Metallkunde, vol. 42, 1951, Dr. Reiderer-VerlagG.m.b.H., Stuttgart, Germany, pages 246- 253, 327-330. Pages 249 and 329relied on.

Nature, December 18, 1948, vol. 162, page 968.

1. FERROMAGNETIC COMPOSITIONS CONSISTING ESSENTIALLY OF 55-70 ATOMPERCENT MAGNESE AND 45-30 ATOM PERCENT GALLIUM.